How to Quickly Identify and Resolve “Address Already in Use” Errors on Linux

What is a listening port

Network ports are identified by a number, an IP address and a protocol (TCP or UDP). A listening port is a port on which an application or process waits for incoming connections, acting as a communication endpoint. A port can be opened or closed by a firewall, and two services cannot listen on the same IP address and port simultaneously.

Using netstat to check listening ports

The netstat command can list all TCP and UDP ports that are in the LISTEN state, together with the owning process. The most common invocation is: sudo netstat -tunlp Options: -t – show TCP ports. -u – show UDP ports. -n – display numeric addresses instead of resolving host names. -l – show only listening sockets. -p – show the PID and program name (requires root).

Sample output:

Proto Recv-Q Send-Q Local Address   Foreign Address   State   PID/Program name
tcp        0      0 0:22               0:*               LISTEN  445/sshd
tcp        0      0 0:25               0:*               LISTEN  929/master
tcp6       0      0 :::3306            :::*               LISTEN  534/mysqld
tcp6       0      0 :::80              :::*               LISTEN  515/apache2
...

Key columns are Proto (protocol), Local Address (IP and port), and PID/Program name. To filter for a specific port, pipe the output to grep, e.g.: sudo netstat -tnlp | grep :22 If the result is empty, no process is listening on that port.

Note: netstat is considered deprecated on many modern systems; ss and ip provide similar functionality.

Using ss to check listening ports

The ss utility is a newer, faster alternative to netstat. Its options are largely compatible: sudo ss -tunlp The output format is similar to netstat and includes the same columns.

Using lsof to check listening ports

lsof

lists open files, and because sockets are treated as files in Unix, it can show listening ports: sudo lsof -nP -iTCP -sTCP:LISTEN Important options: -n – do not resolve host names. -P – do not convert port numbers to names. -iTCP -sTCP:LISTEN – show only TCP sockets in LISTEN state.

Sample output:

COMMAND   PID USER   FD TYPE DEVICE SIZE/OFF NODE NAME
sshd      445 root   3u  IPv4 16434      0t0  TCP *:22 (LISTEN)
apache2   515 root   4u  IPv6 16590      0t0  TCP *:80 (LISTEN)
mysqld    534 mysql  30u IPv6 17636      0t0  TCP *:3306 (LISTEN)
...

To find the process that owns a particular port, e.g. 3306: sudo lsof -nP -iTCP:3306 -sTCP:LISTEN The output shows that the MySQL server (PID 534) is listening on port 3306.